A two-part heat store with large heat-transfer surface area, large thermal mass, and low resistance to air flow

David Delaney
November 14, 2003
Corrected November 19, 2003

This article describes a heat store designed to collect energy from a solar air heater. The air heater is to be located below the heat store, and is to supply warm air to it by natural convection. The intended general arrangement of the solar air heater and the heat store are described in reference [2]. The heat store requires a large thermal mass, a large mass-air surface area for heat transfer, and low resistance to air flow. I propose a heat store having two constituent heat-storing masses. One of the constituents, a very permeable stack of concrete building blocks, has a large concrete-air heat-transfer surface area and a small thermal mass. The other constituent, a stack of drums of water, has a large thermal mass and a small drum-air heat-transfer surface area. Both constituents share a single insulation envelope and operate within a single unpartitioned volume of air.  For carefully chosen circumstances, the performance of the two-component heat store should be similar to the performance of a heat store having the sum of the surface areas and the sum of the thermal masses of the two constituent heat storing masses.  This composite concrete-water heat store requires much less mass and space than a heat store constructed from either concrete or water alone.

During the day the temperature of the concrete blocks will rise faster than the temperature of the water.  During the night, the concrete blocks transfer heat slowly by convection and radiation to the water drums. Because the concrete blocks will give up heat more easily to cool air than will the water drums, the concrete blocks also supply most of the daily heat needs of the building and most of the daily heat losses from the heat store.   As the temperature of the water drum mass rises, the temperature of the concrete block mass cools to meet it,  and becomes ready for efficient absorption of heat from the solar air heater the next day.  (The concrete thermal mass and the ratio of concrete surface to drum surface must be sized to make this true.)  The temperature of the concrete mass will descend below the temperature of the water mass if the night-time heat withdrawals and losses from the heat store are large enough. The water drums will become the main source of heat output only when the temperature of concrete is distinctly less than the temperature of the water.

The exhange of energy between the concrete and the water does, of course, degrade that energy, decreasing the stratification gradient of the heat store. This internal entropy creation will be reasonably limited during a sequence of average sun days, when the temperature of the water drums will remain high and nearly constant.  The greatest rate of entropy creation will occur when the concrete is much hotter than the water drums, as will happen on good sun days following a sequence of very poor sun days.  But in these circumstances, the much greater surface area of the concrete will ensure that much of the high quality heat winds up in the house (if the house needs it) rather than in the water.

Consider a heat store having both a concrete block stack and a water drum stack. Let the concrete block stack have a mass equal to twice the mass of the water, in which case 2/3 of the heat store mass is concrete and 1/3 water. Let the block stack and the drum stack occupy roughly equal areas of the floor of the heat store.

Since concrete has a specific heat close to 1/5 the specific heat of water, and the mass of concrete has twice the mass of the water, the concrete block stack contributes 2/7 of the total thermal mass, and the water contributes 5/7 of the total thermal mass of the heat store.  If a second heat store contains only concrete blocks, and since concrete blocks form 2/3 of the mass of the concrete-water heat store, the mass of the concrete-only heat store can not be less than 7/2*2/3 = 14/6 = 2-1/3 times the mass of the concrete-water heat store if it is to provide at least as much thermal mass as the concrete-water heat store. Since concrete occupies half the floor space of the concrete-water heat store,  the concrete-only heat store must occupy at least 7/2 * 1/2 = 7/4 = 1-3/4 times the floor area of the concrete-water heat store..

The concrete block stack of the concrete-water heat store has 5 times as much surface area in contact with air as the water drum stack of half its mass. (This ratio may be achieved as shown in the detailed design presented below.) The  concrete blocks therefore provide 5/6 of the total heat-transfer surface area of the concrete-water heat store, and the water drums provide 1/6 of the heat-transfer surface area.  If a third heat store were to contain only drums of water, and since water forms 1/3 of the mass of the concrete-water heat store, the mass of the water-only heat store could not be less than 6 *1/3 = 2 times the total mass of the concrete-water heat store if it is to provide the same heat-transfer surface area as the concrete-water heat store. Since water drums occupy half the floor space of the concrete-water heat store, the water-only heat store occupies at least 6*1/2 = 3 times the floor area occupied by the concrete-water heat store.

A suitable concrete block stack, shown below, is described in detail in reference [1].

A suitable water drum stack:
 Here is a tabular presentation of the properties of the concrete block stack, the water drum stack, and their composition as a single thermal mass:
Property Water
Mass 8276 lbm = 3762 kg 19776 lbm = 8989 kg 28052 lbm = 12751 kg
Thermal mass 8276 Btu/F = 15725 kJ/C 3411 Btu/F = 7910 kJ/C 11687 Btu/F = 23636 kJ/C
Surface area 323 ft2 = 30.0 m2 1954 ft2 = 182 m2 2277 ft2 = 212 m2
Floor area 50.4 ft2 = 5.48 m2 59 ft2 = 5.5m2  109.4  ft2 = 11.0 m2
ratio to total
0.29 0.71 1
Thermal mass
ratio to total
0.71 0.29 1
Surface area
ratio to total
0.14 0.86 1
Floor area
ratio to total
0.46 0.54 1

The two stacks described above might be used in combinations of several of each of the two kinds of stack. Such combinations can achieve greater surface area and thermal mass with the same ratio as above, or different ratios of surface area to thermal mass.  Here is the floor plan of a heat store that might be seen in a house built according to the ideas described in reference [2].

The primary consideration in sizing the concrete block stack is the combination of the amount of energy that must be collected in one day and the temperature range of the air from which the energy must be collected.  The desired one-day energy collection divided by the thermal mass of the concrete equals the rise in the average temperature of the concrete required to store that amount of energy. This required temperature rise must be less than the difference between the average temperature of the air from the air heater and the temperature of the concrete at the beginning of the day.

It will be important not to overload the heat store.  An air heater capacity suitable for December in Ottawa will certainly overload the heat store in February. Seasonal shading will have to be provided for the air heater. Some form of emergency heat dumping may have to be provided.


This idea was sugested to me by an article in William Shurcliff's book [3]. The article, scheme S-102, pages 166-168, is titled "How to double the thermal capacity of a bin-of-stones without increasing the size: replace half the stones with an equal volume of water in tanks". Harry Thomason's storage arrangements for his trickle down collectors, described by Shurcliff in another article in the same book [3], were also suggestive.  The article on Thomason's storage arrangement, scheme S-109, pages 184-187, is titled "How to improve the performance of the Thomason storage system". Comments and suggestions by Nick Pine in the alt.solar.thermal newsgroup concerning dual heat stores, one having a "high bandwidth" and the other a "low-bandwidth", were also useful.


[1] A concrete-block thermal mass with large air-concrete surface area, David Delaney, November 2003

[2] Organizing the air flow between a thermosyphon solar air heater and a thermal mass located above it, David Delaney, October 24, 2003

[3] William A. Shurcliff, "New inventions in low-cost solar heating--100 daring schemes tried and untried", p. 184, Brick House Publishing Company, 1979, Andover, Mass

[4] For a significant improvement on the physical arrangement of the two components of the two-part heat store, see Thermal mass of drums of water on top of a stack of concrete blocks, David Delaney, November 23, 2003

For the context of this work, see Solar thermal energy for housing home

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